Plasma processing apparatuses are widely used for manufacturing semiconductor substrates, substrates for a photovoltaic cell, substrates for a display, and other substrates. In order to obtain a substrate for a photovoltaic cell, for example, a silicon-containing plasma is generated above the surface of a glass substrate to deposit a silicon thin film on the glass substrate. Hereinafter, a substrate to which a plasma process is performed will be referred to as a “substrate body to be processed.” (In the aforementioned example, the glass substrate corresponds to the substrate to be processed.)
In recent years, the size of various substrates as previously mentioned has been growing. Such substrates require to be evenly processed all over the surface of one substrate. In the case of a substrate for a photovoltaic cell, for example, one substrate is divided into a plurality of cells. The quality of each cell, such as the thickness of the silicon thin film, must be within a predetermined and limited range. Therefore, it is required that the density distribution of the plasma generated in a plasma processing apparatus should be within a given range, irrespective of the growth in the size of substrate bodies to be processed, or the growth in the size of the plasma production area.
The method of plasma processing apparatuses includes: an electron cyclotron resonance (ECR) plasma method, a microwave plasma method, an inductively coupled plasma method, a capacitively coupled plasma method, and otherwise, For example, Patent Document 1 discloses an inductively-coupled plasma processing apparatus in which a spiral induction coil is placed on the upper surface of the ceiling outside a vacuum chamber. In an inductively coupled plasma processing apparatus, gas is introduced into a vacuum chamber, and a radio-frequency electric current is applied to a radio-frequency antenna (or induction coil) to generate an induction electric field inside the vacuum chamber. This induction electric field accelerate electrons, and then the electrons collide with the gas molecules, so that the gas molecules are ionized to generate a plasma. The plasma processing apparatus described in Patent Document 1 requires an increase in the size of the spiral coil with the growth in the size of substrates. However, simply increasing the size of the spiral coil only increases the difference of the plasma density between the central part and the peripheral part. Accordingly, the criterion of the uniformity over all the surface as previously described cannot be met. In addition, increasing the size of an antenna lengthens the conductor of the antenna, which might form a standing wave in the antenna to create an inhomogeneous intensity distribution of the radio-frequency electric current, resulting in a possible inhomogeneous plasma density distribution (refer to Non-Patent Document 1).
Patent Document 2 and Non-Patent Document 1 disclose multi-antenna inductively coupled plasma processing apparatuses in which a plurality of radio-frequency antennas are attached to the inner walls of a vacuum chamber. In these apparatuses, the plasma distribution in the vacuum chamber can be controlled by appropriately setting the arrangement of the plurality of antennas. In addition, the length of the conductor of each antenna can be short, which can prevent the adverse effect due to the standing wave. For these reasons, the plasma processing apparatuses disclosed by Patent Document 2 and Non-Patent Document 1 can generate a plasma having high uniformity, compared with previous apparatuses.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2000-058297 ([0026]-[0027] and FIG. 1)
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2001-035697 ([0050] and FIG. 11)
[Non-Patent Document 1] Setsuhara Yuichi, “Meter-Scale Large-Area Plasma Sources for Next-Generation Processes,” Journal of Plasma and Fusion Research, vol. 81, no. 2, pp. 85-93, February 2005